As conventional materials of contact and terminal parts, such as connectors and switches, there are used plated products wherein a base material of stainless steel, copper, a copper alloy or the like, which is relatively inexpensive and which has excellent corrosion resistance, mechanical characteristics and so forth, is plated with tin, silver, gold or the like in accordance with required characteristics, such as electrical and soldering characteristics.
Tin-plated products obtained by plating a base material of stainless steel, copper, a copper alloy or the like, with tin are inexpensive, but they do not have good corrosion resistance. Gold-plated products obtained by plating such a base material with gold have excellent corrosion resistance and high reliability, but the costs thereof are high. On the other hand, silver-plated products obtained by plating such a base material with silver are inexpensive in comparison with gold-plated products and have excellent corrosion resistance in comparison with tin-plated products.
As such a silver-plated product, there is proposed a metal plate for electrical contacts, wherein a silver plating film having a thickness of 1 μm is formed on a copper plating film having a thickness of 0.1 to 0.5 μm which is formed on a nickel plating film having a thickness of 0.1 to 0.3 μm which is formed on the surface of a thin base material plate of stainless steel (see, e.g., Japanese Patent No. 3889718). There is also proposed a silver-coated stainless bar for movable contacts, wherein a surface layer of silver or a silver alloy having a thickness of 0.5 to 2.0 μm is formed on an intermediate layer of at least one of nickel, a nickel alloy, copper and a copper alloy having a thickness of 0.05 to 0.2 μm, the intermediate layer being formed on an activated underlying layer of nickel which has a thickness of 0.01 to 0.1 μm and which is formed on the surface of a base material of stainless steel (see, e.g., Japanese Patent No. 4279285). Moreover, there is proposed a silver-coated material for movable contact parts, wherein a surface layer of silver or a silver alloy having a thickness of 0.2 to 1.5 μm is formed on an intermediate layer of copper or a copper alloy having a thickness of 0.01 to 0.2 μm, the intermediate layer being formed on an underlying layer of any one of nickel, a nickel alloy, cobalt or a cobalt alloy which has a thickness of 0.005 to 0.1 μm and which is formed on a metallic substrate of copper, a copper alloy, iron or an iron alloy, and wherein the arithmetic average roughness Ra of the metallic substrate is 0.001 to 0.2 μm, and the arithmetic average roughness Ra after forming the intermediate layer is 0.001 to 0.1 μm (see, e.g., Japanese patent Laid-Open No. 2010-146925).
However, if a silver plating film is formed on the surface of a base material of copper or a copper alloy or on the surface of an underlying layer of copper or a copper alloy formed on a base material in such a conventional silver-plated product, there is a problem in that copper diffuses to form CuO on the surface of the silver plating film to raise the contact resistance thereof if it is used in a high-temperature environment. There is also a problem in that cracks are formed in the silver-plated product to expose the base material if the silver-plated product is worked in a complicated shape or in a shape of small contact and terminal parts, such as connectors and switches. Moreover, there is a problem in that the silver plating film is easily worn.